The following description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
Various processes are known for natural gas liquids (NGL) recovery, and especially for the recovery of propane from high pressure feed gas. At a minimum, hydrocarbon content must be sufficient to meet hydrocarbon dewpoint specifications for pipeline transmission. This generally requires installation of a dewpointing unit that includes a gas-gas exchanger and a refrigeration chiller, and frequently includes ethylene glycol injection exchangers. Ethylene glycol injection typically operates at close to −29° C. (−20° F.), primarily due to the technical challenges of phase separation at lower temperatures. Consequently the propane (i.e. C3) recovery of a dewpointing unit is limited to 30% to 50%, depending upon the feed gas composition.
Liquid products (such as liquid propane) have high value, and there are significant economic incentives to recover C3 as efficiently as possible. As a result there are a number of recovery processes for natural gas liquids (NGL) that utilize a variety of arrangements of heat exchangers, multiple columns, turbo expanders, and complex reflux schemes. The use of turbo expanders and plate fin heat exchangers are currently accepted as standard equipment for NGL recovery unit designs, as shown in U.S. Pat. No. 4,061,481 (to Campbell et al), U.S. Pat. No. 8,590,340 (to Pitman et al), U.S. Pat. No. 7,051,522 (to Mak), and U.S. Patent Application Publication No. 2005/0,255,012 (to Mak). All publications identified herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply. Such plants typically utilize a refluxed absorber operating at low temperatures (at least −51° C. or −60° F.), which are generated using a turbo-expander that reduces the pressure of a chilled, high pressure gas. While effective (producing propane yields of up to 99%), such turbo-expanders are complex devices that represent a significant capital investment and require significant lead time.
Such processes can achieve high C3 recovery, but can only do so if the feed gas flow rate and composition does not deviate significantly from the conditions for which the plant was designed. If there are significant differences from design conditions (for example, suboptimal pressure, suboptimal flow rates, and/or excessively lean gas composition) process inefficiencies can result. For example, if the supplied gas has a leaner composition than is nominal and is supplied at a lower pressure, the brazed aluminum exchangers typically used in such processes can encounter temperature pinches that result in reduced recovery and lower plant throughput. In such a situation the low feed gas pressure reduces the expansion ratio of the turbo-expanders, resulting in reduced cooling effects and lower C3 recovery. Lean gas composition can be caused by upstream nitrogen injection activities used to enhance oil recovery. Typically, leaner gas will lower the temperature profile in the gas chillers, which can exceed the design limits of existing equipment and cause a safety issue. Safe processing of high nitrogen content gas in an existing plant typically requires the use of an expander bypass valve (due to expander capacity limitations), which reduces C3 recovery and plant throughput. In most instances, in order to maintain high C3 recovery under such conditions the impeller of the expander (or in some instances the entire expander) must be replaced. This is not always feasible in small or remote facilities, where supplies and labor may not be readily available.
Typical NGL recovery units utilize brazed aluminum exchangers which can achieve close temperature approaches (less than 4° F.) and high heat transfer efficiency. Such heat exchangers are compact in design and are low in cost (per square foot of heat transfer area) compared to shell and tube exchangers, and have seen widespread adoption in NGL plants. Brazed aluminum exchangers, however, are prone to fouling and damage from mechanical and thermal stress. Aluminum is also a relatively reactive metal and will form amalgams with mercury, even with mercury concentrations in the ppm range. This results in material fatigue and corrosion. In most NGL plants, a mercury removal bed is installed upstream from the NGL recovery unit to protect such aluminum equipment. Aluminum is also prone to thermal stress from high operating temperature, sudden temperature changes, and/or high temperature differentials. A typical aluminum exchanger cannot be operated above 150° F. and temperature differentials between heat exchanger passes must be less than 50° F. Exposure to high temperatures weakens aluminum welds and will result in exchanger failure. As a result, plants utilizing brazed aluminum exchangers require significant operator attention, particularly during startup, shutdown, or whenever temperature excursion is likely.
Almost in all cases, high propane recovery plants require brazed aluminum exchangers and turbo-expander integrated with complex heat exchange configurations, multiple columns and various refluxes. Such brazed aluminum exchangers are prone to stress failure, and while turbo-expander(s) can be utilized to improve recovery efficiency and reduce energy consumption, optimal performance of such devices is limited to the design flow rate. Rotating equipment such as the expander-compressors used in current NGL recovery processes is limited to a turndown rate of approximately 60%. Below this turndown rate, the expander has to be shut down, and the unit operated in a JT valve (i.e. bypass) mode. Under such circumstances NGL recovery is significantly reduced.
In current shale gas exploration the resulting feed gas compositions and flow rates are uncertain. As a result there are inherent design difficulties with the traditional plant designs for NGL recovery from such sources. To accommodate these uncertainties typical mid-stream processors are forced to employ multiple turbo-expander units to accommodate the inevitable variations in turndown gas flow and gas composition. While such an approach can achieve basic process requirements, the use of multiple turbo-expander units significantly increases design complexity, capital costs, and maintenance requirements.
Current high C3 recovery processes, with their high equipment counts and requirement for experienced and highly skilled staff, are not a suitable choice for shale-gas NGL plants or plants located in remote locations. While numerous attempts have been made to improve the efficiency and economy of processes for separating and recovering ethane, propane, and heavier natural gas liquids from natural gas and other sources, all or almost all of them suffer from one or more disadvantages. Most significantly, heretofore known configurations and methods are configured for very high C3 recovery with complex design.
Thus there remains a need for simple and robust systems and methods that permit highly efficient recovery of C2 and C3 NGL fractions when supplied with a broad range of feed gas compositions and pressures.